Recently, a fuel cell, which directly converts chemical energy of fuel into electric energy, has drawn attention as a clean and efficient power generating device. Especially, the solid oxide fuel cell has the advantages that its power generation efficiency is high and exhaust heat can be utilized effectively, therefore, it has been developed as a third generation fuel cell for power generation. The solid oxide fuel cell has a laminated structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode (cathode) layer and a fuel electrode (anode) layer. At the time of power generation, oxidant gas (air) is supplied to the air electrode side, and fuel gas (H2, CO, CH4 or the like) is supplied to the fuel electrode side, as reactant gases. Both the air and fuel electrodes are made porous so that the reactant gases can reach their boundary with the solid electrolyte.
In the power generation cell, the oxygen supplied to the air electrode layer side reaches near the boundary with the solid electrolyte layer through the pore in the air electrode layer, and there, the oxygen receives an electron from the air electrode layer to be ionized to oxide ion (O2−). The oxide ion is diffusively moved in the solid electrolyte layer toward the direction of the fuel electrode layer. When reaching near the boundary with the fuel electrode layer, the oxide ion reacts there with fuel gas to produce reaction products (H2O, CO2 and the like), and emits an electron to the fuel electrode layer. The electrons obtained by the electrode reaction are taken out as an electromotive force by an external load on another route.
The flat plate laminated type solid oxide fuel cell is constructed by alternately laminating power generation cells and separators to form a stack structure; and applying load in the laminating direction from both ends of the stack so that elements of the stack are pressure bonded and closely overlapped to each other.
The separator has a function of electrically connecting the power generation cells to each other and of supplying reactant gas to the power generation cell, and is provided with a fuel gas passage which introduces fuel gas to the fuel electrode layer side, and with an oxidant gas passage which introduces oxidant gas to the air electrode layer side.
As configurations for supplying external reactant gases to the separators, the following systems are known: an external manifold system in which an external manifold is provided on the circumference of the fuel cell stack and each gas is supplied to each of the separators through a plurality of connecting pipes; and an internal manifold system in which gas openings are formed on the peripheral portion of each separator and fuel gas and oxidant gas are supplied from the gas openings to each electrode surface of the power generation cell through the gas passages (See Patent Document 1). In the internal manifold system, the gas openings of any two adjacent separators are in communication with each other through a ring-shaped insulating gasket (spacer) interposed between the separators.
In the flat plate laminated type solid oxide fuel cell using the internal manifold system, it is generally known that each reactant gas is introduced from one end side of each manifold and is distributed and supplied into each of the separators through the gas openings of the separators in the process of flowing within the manifold in the laminating direction.
However, in such a configuration, there will be differences in gas pressure between the near (or, proximal) side of the gas inlet (that is, the upstream) and the far (or, distal) side (the downstream) in the manifold, the difference being variations of the differential pressure in a longitudinal direction of the manifold. Thus, there is a tendency that the pressure in the manifold is higher in the proximal side of the gas inlet and lower in the distal side. Therefore, gas volume flowing into the power generation cells located upstream of the gas flow will be increased, on the other hand, gas volume flowing into the power generation cells located downstream of the gas flow will be decreased.
Consequently, the gas volume distributed to each power generation cell becomes nonuniform, as a result, the output voltage of the power generation cells located downstream will be decreased due to deficient gas supply, and this causes deterioration of the fuel cell performance of the stack as a whole. This phenomenon becomes increasingly prominent as the number of layers of the stack increases.
Patent Document 1: Japanese Patent Laid-Open No. 7-201353